1. Field of the Technology
The present invention relates to a rotation control apparatus for an electric motor, especially an electronic commutator motor of very accurately smooth revolution.
In recent years, small type, high efficiency and reliable DC electronic commutator motors become widely used as rotary heads driving motor, capstan driving motor or reel driving motor of video-tape recorder (hereinafter is referred to as VTR) of inclined scanning tape type wherein recording and reproduction of video signal are made by rotary heads, by direct coupling of such electronic commutator motor to the rotary head cylinder, capstan shaft and reel shaft. In such VTR, since the video signal is recorded by the rotary heads driven at a very high speed by the rotary head driving motor on a magnetic recording media, if there are some non-uniformity of rotation takes place the time axis of the video signal fluctuates, and therefore the reproduced video image has horizontal vibration resulting in bad quality of picture. Accordingly, torque ripple of rotary head driving motor which becomes undesirable rotation fluctuation must be controlled in order to eliminate such undesirable deterioration of the picture.
Furthermore, the audio signal is generally recorded in the VTR by means of a fixed audio recording head which is disposed fixedly with respect to the running tape and to the rotating video head. Accordingly, when a capstan driving motor which drives the video tape makes a fluctuation of rotation speed, then the reproduced audio signal has undesirable distortions called wow and flutter induced by fluctuation of time axis of the audio recording. Accordingly, in the capstan driving motor, torque ripple which becomes the cause of the fluctuation of rotation must be controlled.
Furthermore, also in the reel driving motor, unless torque ripple produced thereby is suppressed, the video tape of running state is subject to fluctuation of tape tension, and thereby induces fluctuation of rotations of capstan driving motor and rotary head driving motor, resulting in the undesirable horizontal vibration of the reproduced picture, and wow and flutter of the reproduced audio signal.
As is elucidated above, in order to obtain stable and good reproduced signal of the VTR or the like apparatus, the suppressing of torque ripple of motor becomes very important item.
Hitherto, suppressing of the fluctuation of rotation induced by the torque fluctuation of the motor has been made by increasing moment of inertia of the rotating members and increase of gains of controlling circuit of the motor.
However, in portable type VTRs, in order to decrease weight and size, moment of inertia of the driving motors of the rotary heads and the capstan must be made small. Moreover, when direct-couple driving are adopted for such driving motors, the rotation speed of the motor becomes low, and therefore the hitherto-adopted way of relying on the increased moment of inertia and increased gain can not satisfactorily suppress the ripple of rotations of the related members.